Unless she needs them to protect her feet, never. In the words of the AAP: "Style is the only reason for a baby to wear shoes at all until the child begins walking outdoors or is taken out in cold weather."
First, the idea that her feet will get too big is somewhere between ignorant and harmful. Wearing correctly-sized shoes will not prevent foot growth. Wearing shoes that are too small WILL retard foot growth in entirely bad ways.
The American Association of Pediatrics states there's no need to wear shoes other than for protection and that walking without them allows your child to use her toes for gripping and balance.
As far as altering foot growth, here's the AAP article warning of improper shoe use.
The foot takes the shape of the shoe, not vice versa. Improperly fitted or manufactured shoes may be the primary cause of acquired foot deformities and problems. Shoes that do not fit properly can deform an otherwise-normal foot, resulting in hammertoes, hallux valgus, bunionettes, corns, and, ultimately, the need for surgery.
Make note: other studies have indicated that even putting your child in used shoes can be problematic, as the shoe will have become deformed to varying extents by the previous wearer. That alteration can then be passed on to your child's feet.
As far as the habit of wearing shoes, I'm not sure I believe it's likely to make a difference. Some kids can be acclimated to things they dislike, some will fight you to the end. However, in the interest of grandparent harmony, why not let them buy her some shoes? You will surely take her places where she needs footwear to walk around so having some won't be harmful. Just make sure they are properly fitted and you move on to the next size as needed.